1. Field of the Invention
The present invention relates to a control apparatus for driving a powder feeder, through which feed amount of powder can be precisely controlled within a short time from the start of powder feed, and relates to a powder feeder in which such control apparatus installed.
2. Description of Related Art
It is conventionally well-known a powder feeder in which an ultrasonic motor having an ultrasonic vibrator is utilized. In the ultrasonic vibrator, it is used a piezoelectric element which is mechanically deformed when electrical energy is applied thereto. The powder feeder feeds the powder by driving the ultrasonic vibrator through applying a driving voltage with a resonance frequency and producing mechanical vibration in the ultrasonic vibrator through the piezoelectric element. That is, in the powder feeder, if an AC driving voltage with the resonance frequency is applied to the ultrasonic motor which is constructed so as to simultaneously produce axial vibration (longitudinal vibration) and bending vibration, elliptic oscillation is produced at the top end of the vibrator. Based on this, a feed pipe is attached at the top end of the vibrator, and the powder supplied into the feed pipe is fed in the certain direction on the basis of the elliptic oscillation of the vibrator.
The feed amount of the powder is controlled as follows. Namely, the driving voltage is intermittently applied to the ultrasonic motor. At that time, output of the driving voltage to the ultrasonic motor is changed by (changing proportion of the driving voltage per one cycle thereof (duty ratio). Thereby, the feed amount of the powder is controlled. For example, in order to perform such control, the output from a resonance frequency oscillation circuit which is oscillating with the resonance frequency and the output from the duty ratio control clock generation means for changing the duty ratio are supplied to an AND (product) circuit to generate the product, and the product output is amplified and applied to the vibrator.
Further, in the powder feeder, feedback control is conducted by using the control system shown in FIG. 5, to improve accuracy of feed amount control of the powder. In the control system, powder amount is detected by a load cell acting as a load sensor and the sensor output signal is amplified in a load cell amplifier. The amplified output signal is fed back to an A/D converter, thereafter the output signal is sampled by a microcomputer system as duty ratio control means through the A/D converter. In the microcomputer system, a duty ratio clock signal is calculated on the basis of the amplified output signal and a drive circuit receives the duty ratio clock signal. Thereafter, a driving voltage signal is applied to the ultrasonic motor, thereby the ultrasonic motor is driven. In this way, since the microcomputer system calculates the optimum duty ratio based on the output signal from the load cell, the feed amount of the powder can be properly controlled.
However, in the above powder feeder, though the feedback control is conducted so as to coincide the actual feed amount of the powder with the target feed amount, there is a problem that it takes a long time until the actual feed amount becomes the target feed amount. For instance, as shown in FIG. 13, it takes the time T.sub.2 (several seconds) including the response time of the control system until the actual feed amount becomes the target feed amount Q.sub.T from the start of powder feeding. Therefore, the actual feed amount becomes very unstable and thus the powder cannot be supplied with the target feed amount Q.sub.T during it passes several seconds from the start of powder feeding. The reason is due to that feed control of the powder until the feed amount reaches the target feed amount Q.sub.T is conducted by gradually increasing the duty ratio every time that the powder feed is done.
Thus, at the initial of the powder feed, the actual feed amount largely deviates from the target feed amount Q.sub.T, as a result, the powder cannot be precisely fed with the predetermined amount. Due to this, during a short time, for example, several seconds from the start of powder feed, there exists a large error between the actual feed amount and the target feed amount Q.sub.T, and there remains a problem that the powder cannot be precisely fed with a predetermined amount.
Further, since flow characteristic of the powder is different due to fluctuation in properties or lots of the powder, the feed accuracy of the powder is seriously influenced by such fluctuation. Thus, in order to retain the feeding accuracy, it is necessary to conduct the feeder calibration every time properties or lots of the powder change, thus it is very troublesome.